Calorie to Kilojoule Converter
Convert calories to kilojoules with our free online energy converter.
Quick Answer
1 Calorie = 0.004184 kilojoules
Formula: Calorie × conversion factor = Kilojoule
Use the calculator below for instant, accurate conversions.
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Calorie to Kilojoule Calculator
How to Use the Calorie to Kilojoule Calculator:
- Enter the value you want to convert in the 'From' field (Calorie).
- The converted value in Kilojoule will appear automatically in the 'To' field.
- Use the dropdown menus to select different units within the Energy category.
- Click the swap button (⇌) to reverse the conversion direction.
How to Convert Calorie to Kilojoule: Step-by-Step Guide
Converting Calorie to Kilojoule involves multiplying the value by a specific conversion factor, as shown in the formula below.
Formula:
kJ = cal × 0.004184Example Calculation:
1 cal = 0.004184 kJ
Disclaimer: For Reference Only
These conversion results are provided for informational purposes only. While we strive for accuracy, we make no guarantees regarding the precision of these results, especially for conversions involving extremely large or small numbers which may be subject to the inherent limitations of standard computer floating-point arithmetic.
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and Standards
The Small Calorie (Thermochemical Calorie)
The small calorie (cal), also called the gram calorie or thermochemical calorie, is defined as:
1 cal = 4.184 joules (exactly)
This represents the energy required to raise the temperature of one gram of pure water by one degree Celsius (from 14.5°C to 15.5°C) at standard atmospheric pressure (101.325 kPa).
The Large Calorie (Kilocalorie)
The large Calorie (Cal with capital C), properly called the kilocalorie (kcal), is defined as:
1 Cal = 1 kcal = 1,000 cal = 4,184 joules = 4.184 kilojoules
This represents the energy required to raise one kilogram of water by one degree Celsius. Food labels universally use this unit but write it as "Calorie" or "calories" (often lowercase), causing widespread confusion.
Historical Calorie Definitions
Multiple calorie definitions existed historically, all slightly different:
- 15°C calorie (cal₁₅): Energy to raise 1 g water from 14.5°C to 15.5°C = 4.1855 J
- 20°C calorie (cal₂₀): Energy to raise 1 g water from 19.5°C to 20.5°C = 4.182 J
- Mean calorie (cal_mean): 1/100 of energy to raise 1 g water from 0°C to 100°C = 4.190 J
- International Table calorie (cal_IT): Exactly 4.1868 J (defined in 1956)
- Thermochemical calorie (cal_th): Exactly 4.184 J (most common today)
The thermochemical calorie (4.184 J) is now standard in most applications.
Modern SI Replacement
The calorie was officially replaced by the joule (J) as the SI unit of energy in 1948. However, nutrition labeling and food science continue using kilocalories (Calories) globally, creating a dual-system reality.
and Standards
The kilojoule is defined as exactly 1,000 joules:
1 kJ = 1,000 J = 1,000 kg·m²/s² = 1,000 N·m
Precise Equivalents
- 1 kJ = 1,000 J (exactly, by definition)
- 1 kJ = 0.239006 kilocalories (kcal, food Calories)
- 1 kJ = 0.947817 British thermal units (BTU)
- 1 kJ = 0.000277778 kilowatt-hours (kWh)
- 1 kJ = 6.242 × 10²¹ electronvolts (eV)
- 1 kcal = 4.184 kJ (exactly, thermochemical definition)
SI System Integration
The kilojoule is a standard SI unit formed by applying the metric prefix kilo- (1,000) to the base unit joule. Unlike the calorie, which has multiple definitions and creates confusion between small calories (cal) and large Calories (kcal), the kilojoule provides unambiguous measurement consistent across all scientific disciplines.
Nutrition Labeling Standards
International practice:
- Australia, New Zealand: kJ only (mandatory since 1995/2003)
- European Union: Both kJ and kcal required (kJ first, since 2016)
- China, Japan, South Korea: Primarily kJ with optional kcal
- Canada: Both kJ and kcal required
- United States: kcal only (Calories)
Standard format: "Energy: 250 kJ (60 Cal)" or "Energy: 1,000 kJ / 239 kcal"
Note: The Calorie is part of the imperial/US customary system, primarily used in the US, UK, and Canada for everyday measurements. The Kilojoule belongs to the imperial/US customary system.
History of the Calorie and Kilojoule
and Evolution
Early Thermal Theory (1770s-1820s)
Swedish physicist Johan Wilcke first used the term "calorie" in 1772 while studying the specific heat capacity of different substances. French chemist Antoine Lavoisier and mathematician Pierre-Simon Laplace developed early calorimetry techniques in the 1780s using ice calorimeters, though they didn't use the calorie unit.
French engineer Nicolas Clément formally defined the calorie in 1824 as the heat required to raise one kilogram of water by one degree Celsius (equivalent to the modern kilocalorie). This definition emerged from steam engine efficiency studies during the Industrial Revolution.
Refinement and Standardization (1840s-1890s)
French chemists Pierre-Antoine Favre and Johann Silbermann refined Clément's definition in the mid-19th century to the small calorie (one gram of water by one degree Celsius), making it more practical for laboratory thermochemistry measurements.
German physicist Julius von Mayer and British physicist James Prescott Joule established the mechanical equivalent of heat in the 1840s-1850s, showing that heat and mechanical work were forms of the same phenomenon—energy. Joule's experiments demonstrated that approximately 4.184 joules of mechanical work produced one calorie of heat.
By the 1880s, the calorie became fundamental to thermochemistry, with chemists using it to measure heats of combustion, reaction, and formation for thousands of compounds.
Nutrition Science Revolution (1890s-1910s)
American agricultural chemist Wilbur Olin Atwater pioneered the use of calories in nutrition science in the 1890s. Working at Wesleyan University and the USDA, Atwater:
- Built the first human respiration calorimeter in the United States (1892)
- Measured the caloric content of hundreds of foods
- Established the Atwater system: 4 kcal/gram for carbohydrates and proteins, 9 kcal/gram for fats, 7 kcal/gram for alcohol
- Published the first food composition tables with calorie values (1896)
Atwater used the kilocalorie (large Calorie) for practical reasons—food portions contain thousands of small calories. His system remains the foundation of modern nutrition labeling.
Global Nutrition Labeling (1960s-Present)
The United States mandated calorie labeling on packaged foods in 1990 through the Nutrition Labeling and Education Act (NLEA), with regulations taking effect in 1994. The European Union, Australia, Canada, and most other nations followed with similar requirements.
International standards bodies attempted to introduce joule-based labeling:
- EU regulations required joules (kJ) alongside Calories (kcal) starting in the 1990s
- Australian/New Zealand labels show both kJ (primary) and Cal (secondary)
- Most other nations continue using Calories (kcal) exclusively
Despite official SI adoption of the joule, the Calorie remains dominant in consumer nutrition worldwide due to decades of established practice.
and Evolution
The Joule Foundation: Mechanical Equivalent of Heat (1840s-1850s)
The kilojoule's origins trace to James Prescott Joule's groundbreaking experiments establishing that mechanical work and heat are equivalent forms of energy. His famous paddle-wheel experiments (1843-1850) demonstrated that a specific amount of mechanical work always produces the same amount of heat, establishing the mechanical equivalent of heat: approximately 4.184 joules per calorie.
This work unified thermodynamics and mechanics, providing the foundation for energy measurement in consistent units rather than the historically separate "calories" for heat and "work" for mechanical energy.
SI Adoption and the Joule as Standard (1960)
The International System of Units (SI) adopted the joule as the standard unit of energy in 1960, derived from the base units kilogram, meter, and second. The kilojoule naturally followed as the convenient multiple for everyday energy measurements, particularly for food and human metabolism where joule values would be inconveniently large (a typical meal contains millions of joules but only thousands of kilojoules).
International Nutrition Standards: WHO and FAO (1970s-1980s)
The World Health Organization (WHO) and Food and Agriculture Organization (FAO) began recommending kilojoules for international nutrition standards in the 1970s-1980s. Reports on dietary energy requirements transitioned from kilocalories to kilojoules to align with scientific practice and eliminate confusion between calories and Calories.
Key milestone: The 1985 FAO/WHO/UNU Expert Consultation on Energy and Protein Requirements recommended expressing all energy values in both kilojoules and kilocalories, with kilojoules as the primary unit.
Australia: First Mandatory kJ Labeling (1995)
Australia became the first country to mandate exclusive kilojoule labeling on packaged foods in 1995, eliminating Calories entirely from nutrition panels. This bold move aimed to:
- Align consumer nutrition information with scientific standards
- Eliminate calorie/Calorie confusion
- Educate the public in metric energy units
- Standardize with international scientific practice
Reaction: Initial consumer confusion gave way to acceptance within 5-10 years. By 2005, surveys showed most Australians understood kilojoule values and could estimate daily requirements in kJ.
Menu labeling: In 2011, Australia extended kilojoule labeling to chain restaurant menus, requiring displayed kJ values for all items (but not Calories).
New Zealand and EU Adoption (2000s-2010s)
New Zealand followed Australia with mandatory kilojoule-only labeling in 2003.
The European Union required both kilojoules and kilocalories on nutrition labels from 2016 (under Regulation EU 1169/2011), with kilojoules listed first to emphasize the SI unit while maintaining familiarity for consumers accustomed to Calories.
United States: The Calorie Holdout
The United States remains the primary developed nation using kilocalories (Calories) exclusively on nutrition labels. The FDA updated nutrition fact panels in 2016 and 2020 without adopting kilojoules, citing consumer familiarity and the impracticality of changing established practice.
Scientific paradox: US nutrition scientists, exercise physiologists, and researchers routinely use kilojoules in academic publications while consumers encounter only Calories on products.
Global Status (2025)
Countries using kJ exclusively: Australia, New Zealand, Singapore (primary)
Countries using both kJ and kcal: European Union, Canada, China, Japan, South Korea, India, Brazil, Argentina, South Africa
Countries using kcal exclusively: United States, Myanmar
Population coverage: Over 85% of the world's population lives in countries where kilojoules appear on nutrition labels, either exclusively or alongside kilocalories.
Common Uses and Applications: calories vs kilojoules
Explore the typical applications for both Calorie (imperial/US) and Kilojoule (imperial/US) to understand their common contexts.
Common Uses for calories
Nutrition and Food Science
The kilocalorie (food Calorie) is the global standard for food energy labeling:
- Nutrition Facts labels: Required in US, EU, Canada, Australia, Japan, and most nations
- Calorie counting apps: MyFitnessPal, Lose It!, Cronometer use kcal
- Restaurant menus: Chain restaurants in many countries must display Calories
- Weight management: "3,500 Calorie deficit = 1 pound lost" rule (approximate)
- Dietary guidelines: USDA recommends 2,000 Cal/day reference (individual needs vary)
- Meal planning: Registered dietitians prescribe meal plans in kcal targets
The capital-C "Calorie" convention (Cal) distinguishes food calories from scientific small calories (cal), though lowercase "calorie" appears on many labels causing confusion.
Thermochemistry and Chemical Engineering
The small calorie (cal) remains common in chemistry despite SI joule adoption:
- Heat capacity measurements: Specific heat often expressed in cal/(g·°C)
- Enthalpy of reaction: ΔH values published in both kcal/mol and kJ/mol
- Bond energies: C-H bond ≈ 99 kcal/mol (414 kJ/mol), C-C bond ≈ 83 kcal/mol (347 kJ/mol)
- Calorimetry: Bomb calorimeters measure heat of combustion in cal or kcal
- Physical chemistry textbooks: Many retain calories alongside joules for historical continuity
Exercise Physiology and Sports Science
Calories (kcal) measure energy expenditure and athletic performance:
- VO₂ max testing: Maximum oxygen consumption converted to Cal/min
- Heart rate monitors: Estimate calorie burn from HR, age, weight, gender
- Fitness trackers: Apple Watch, Fitbit, Garmin display "active Calories" and "total Calories"
- Exercise equipment: Treadmills, ellipticals, rowing machines show estimated Calories burned
- Sports nutrition: Athletes track calorie intake to match training expenditure (3,000-8,000+ Cal/day for elite athletes)
Medicine and Clinical Nutrition
Healthcare providers use Calories for patient care:
- Enteral/parenteral nutrition: Hospital feeding formulas prescribed in kcal/day
- Malnutrition treatment: Refeeding protocols specify gradual kcal increases
- Obesity medicine: Weight loss interventions target 500-1,000 Cal/day deficits
- Diabetes management: Carbohydrate counting considers total caloric intake
- Critical care: ICU patients receive calculated kcal needs (25-30 kcal/kg body weight/day)
- Eating disorder treatment: Recovery meal plans gradually increase kcal to restore weight
Heating and Cooling Systems (Historical)
Before universal adoption of BTUs and watts, calories measured heating/cooling:
- Steam heating systems: Rated in thousands of kcal/hour (now watts or BTU/h)
- Refrigeration: Early systems rated in kcal/hour removed (now tons or watts)
- Industrial processes: Heat input/output in Mcal (megacalories = 1 million cal = 1,000 kcal)
When to Use kilojoules
1. Nutrition Labeling and Dietary Planning
The kilojoule is the international standard for expressing food energy content on nutrition labels, enabling consumers to track energy intake and manage weight.
Typical nutrition panel (Australia/NZ):
Nutrition Information
Servings per package: 5
Serving size: 30g
Per Serving Per 100g
Energy 600 kJ 2,000 kJ
Protein 3.0 g 10.0 g
Fat, total 5.0 g 16.7 g
- saturated 2.5 g 8.3 g
Carbohydrate 20.0 g 66.7 g
- sugars 5.0 g 16.7 g
Sodium 150 mg 500 mg
Weight management: To lose 0.5 kg per week, create a deficit of ~2,000 kJ (500 kcal) per day through reduced intake and increased activity.
2. Exercise Physiology and Sports Nutrition
Exercise scientists use kilojoules to quantify energy expenditure during physical activity and calculate fuel requirements for athletes.
Metabolic equivalent tasks (METs):
- 1 MET = resting metabolic rate ≈ 4.2 kJ per kg body weight per hour
- Moderate exercise = 3-6 METs = 12.6-25.2 kJ/kg/hr
- Vigorous exercise = 6+ METs = 25.2+ kJ/kg/hr
Example: A 70 kg person running at 8 METs for 1 hour:
- Energy = 70 kg × 8 METs × 4.2 kJ/kg/hr = 2,352 kJ (562 kcal)
3. Chemistry: Reaction Enthalpies
Chemists use kilojoules to express heat of reaction (ΔH), combustion energies, and bond energies.
Examples:
- Combustion of methane: CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(l), ΔH = -890 kJ/mol
- Photosynthesis: 6CO₂ + 6H₂O + 2,870 kJ → C₆H₁₂O₆ + 6O₂
- Cellular respiration: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + 2,870 kJ
Bond energies (typical values in kJ/mol):
- C-H: 413 kJ/mol
- C-C: 348 kJ/mol
- C=C: 614 kJ/mol
- C≡C: 839 kJ/mol
- O-H: 463 kJ/mol
- N-H: 391 kJ/mol
4. Thermodynamics and Heat Transfer
Engineers use kilojoules for heat transfer calculations, thermal energy storage, and efficiency analysis.
Specific heat capacity (energy to raise 1 kg by 1°C):
- Water: 4.18 kJ/(kg·°C)
- Air: 1.01 kJ/(kg·°C)
- Aluminum: 0.90 kJ/(kg·°C)
- Concrete: 0.88 kJ/(kg·°C)
Example: Heating 50 liters (50 kg) of water from 20°C to 60°C:
- Energy = 50 kg × 4.18 kJ/(kg·°C) × (60-20)°C = 8,360 kJ (2.32 kWh)
5. Climate and Environmental Science
Climate scientists use kilojoules to quantify solar radiation, atmospheric energy budgets, and heat storage in oceans.
Solar irradiance: ~1,000 watts per square meter at Earth's surface = 3,600 kJ/m²/hr
Ocean heat content: Oceans store vast energy measured in zettajoules (10²¹ J), but marine biology and oceanography often express smaller values in kJ (e.g., energy available to marine organisms).
6. Consumer Energy Awareness
Kilojoule labeling on restaurant menus (mandatory in Australia, parts of US) helps consumers make informed choices.
Example chain restaurant menu (with kJ):
- Grilled chicken salad: 1,500 kJ
- Cheeseburger with fries: 4,200 kJ
- Vegetarian wrap: 1,800 kJ
- Large soft drink: 800 kJ
Daily context: If your target is 8,400 kJ/day, the cheeseburger meal consumes exactly half your daily budget.
Additional Unit Information
About Calorie (cal)
What is the difference between cal and kcal?
cal (small calorie) = 4.184 joules—the energy to raise 1 gram of water by 1°C. Used in scientific contexts (chemistry, physics).
kcal (kilocalorie) = 4,184 joules = 1,000 cal—the energy to raise 1 kilogram of water by 1°C. Used in nutrition as "Calorie" (capital C).
Food labels display kilocalories (kcal) but call them "Calories" or "calories," causing widespread confusion.
Why do food labels use "Calories" instead of "calories"?
Food labels show the kilocalorie (kcal), which should technically be written as "Calorie" with capital C or "kcal" to distinguish from the small calorie (cal). However, regulations and common usage often write "calories" (lowercase), relying on context (nutrition labeling) to indicate kilocalories.
Historical reason: When Wilbur Atwater introduced food calorie measurements in the 1890s, he used kilocalories because food portions contain thousands of small calories. The "Calorie" (capital C) convention emerged to distinguish the two.
How many calories do I need per day?
Estimated daily needs (kcal/day):
- Sedentary women: 1,600-2,000 Calories
- Active women: 2,000-2,400 Calories
- Sedentary men: 2,000-2,600 Calories
- Active men: 2,400-3,000 Calories
Factors affecting needs:
- Age (decreases ~10 Cal/day per year after age 30)
- Sex (men typically need more due to higher muscle mass)
- Height and weight (larger bodies need more energy)
- Activity level (exercise increases expenditure significantly)
- Muscle mass (muscle burns more calories at rest than fat)
Use the Mifflin-St Jeor equation for personalized estimates:
- Men: (10 × weight in kg) + (6.25 × height in cm) - (5 × age) + 5
- Women: (10 × weight in kg) + (6.25 × height in cm) - (5 × age) - 161
Then multiply by activity factor (1.2 for sedentary, 1.375 for light activity, 1.55 for moderate, 1.725 for very active, 1.9 for extremely active).
How many calories should I eat to lose weight?
Safe weight loss rate: 0.5-2 pounds per week
Calorie deficit needed:
- 500 Cal/day deficit → ~1 pound lost per week
- 1,000 Cal/day deficit → ~2 pounds lost per week
Minimum intake guidelines:
- Women: Don't go below 1,200 Calories/day without medical supervision
- Men: Don't go below 1,500 Calories/day without medical supervision
Why minimums matter: Very low-calorie diets risk nutrient deficiencies, muscle loss, metabolic slowdown, and difficulty sustaining weight loss.
Better approach: Calculate your Total Daily Energy Expenditure (TDEE), then eat 10-25% less while maintaining adequate protein (0.7-1g per pound body weight) and strength training to preserve muscle mass.
Are calories the same as kilojoules?
No, they are different units measuring the same thing (energy):
1 Calorie (kcal) = 4.184 kilojoules (kJ)
Conversion:
- kcal → kJ: multiply by 4.184
- kJ → kcal: divide by 4.184
Regional use:
- United States, Canada: Calories (kcal) exclusively
- Australia, New Zealand: kJ primary, Calories secondary
- European Union: Both required (kJ first in some countries)
- Most of world: Calories (kcal) dominant
Quick approximation: 1 kJ ≈ 0.24 kcal, or 1 kcal ≈ 4.2 kJ
Why was the calorie replaced by the joule?
The joule became the SI standard unit of energy in 1948 for several reasons:
- Coherence: Joule integrates perfectly with other SI units (1 J = 1 kg·m²/s²)
- Universality: Applies to all forms of energy (mechanical, electrical, thermal), not just heat
- Precision: Joule has an exact definition based on fundamental constants
- No ambiguity: Multiple calorie definitions existed (15°C, 20°C, mean, thermochemical, IT)
Despite official replacement, the calorie persists in nutrition because:
- Decades of established practice and consumer familiarity
- Food databases and recipes use Calories
- Changing would require massive re-education and relabeling costs
- The kilocalorie provides convenient numbers for food portions (100-500 range vs. 400,000-2,000,000 joules)
Do different foods have different calorie densities?
Yes, calorie density varies enormously:
Very low calorie density (0-50 Cal/100g):
- Non-starchy vegetables: lettuce 15 Cal/100g, cucumber 16 Cal/100g, celery 16 Cal/100g
- Fruits: watermelon 30 Cal/100g, strawberries 32 Cal/100g
Low calorie density (50-150 Cal/100g):
- Lean proteins: chicken breast 165 Cal/100g, cod 82 Cal/100g
- Legumes: lentils 116 Cal/100g cooked, black beans 132 Cal/100g cooked
Medium calorie density (150-400 Cal/100g):
- Whole grains: brown rice 123 Cal/100g cooked, whole wheat bread 247 Cal/100g
- Starchy vegetables: potatoes 77 Cal/100g, corn 96 Cal/100g
High calorie density (400-900 Cal/100g):
- Fats and oils: olive oil 884 Cal/100g, butter 717 Cal/100g
- Nuts and seeds: almonds 579 Cal/100g, peanut butter 588 Cal/100g
- Processed foods: potato chips 536 Cal/100g, chocolate 546 Cal/100g
Why fat has highest density: Fat provides 9 Cal/g vs. 4 Cal/g for carbs/protein, making fatty foods extremely calorie-dense.
Does "negative calorie" food exist?
No, truly negative-calorie foods do not exist. All foods provide calories, though some extremely low-calorie vegetables come close.
The myth: Foods like celery supposedly require more calories to digest than they provide.
The reality:
- Celery contains 16 Calories per 100g (about 6 Calories per large stalk)
- Digestion burns ~10-30% of calories consumed (thermic effect of food)
- Celery digestion might use ~2 Calories per stalk
- Net result: Still positive 4 Calories per stalk
Closest candidates (still positive net calories):
- Celery, cucumbers, lettuce, zucchini (very low but never negative)
- Ice water technically uses ~8 Calories per liter to warm (negligible effect)
Bottom line: Very low-calorie vegetables are excellent for weight loss due to high volume and fiber, but they still provide net positive calories.
How accurate are calorie counts on food labels?
FDA allows ±20% margin of error on nutrition labels in the United States. Similar tolerances exist in other countries.
Why inaccuracy occurs:
- Natural variation in food composition (seasonal, variety, growing conditions)
- Measurement methods (Atwater system uses averages, not exact values)
- Rounding rules (labels round to nearest 5 or 10 Calories)
- Fiber calories counted differently (2 Cal/g vs. 4 Cal/g for total carbs)
Studies show:
- Restaurant foods average 18% more calories than posted
- Packaged foods usually within ±10% of label
- "Low calorie" foods (<40 Cal/serving) often more accurate in absolute terms
Calorie counting app accuracy: Database entries vary widely; verified entries (e.g., from USDA) are most reliable, while user-submitted entries can be off by 50%+ in some cases.
What is a "calorie bomb calorimeter"?
A bomb calorimeter is a laboratory device that precisely measures the energy content of food and other materials by burning them completely in pure oxygen and measuring the heat released.
How it works:
- Sample placed in sealed steel "bomb" container filled with pure oxygen (30 atm pressure)
- Bomb submerged in precisely measured water
- Electrical ignition burns sample completely
- Temperature rise in surrounding water measured
- Calories calculated: Heat (cal) = water mass (g) × temp rise (°C) × water specific heat
Accuracy: Modern bomb calorimeters measure to ±0.1% precision.
Why important: Food calorie values originally came from bomb calorimetry. However, human digestion doesn't extract 100% of calories (fiber partly undigested, protein incompletely oxidized), so Atwater factors adjust bomb calorimetry values for physiological availability.
Can you burn calories without exercise?
Yes, your body burns calories constantly through:
1. Basal Metabolic Rate (BMR): 60-75% of daily calories
- Cellular metabolism, protein synthesis, breathing, circulation, brain function
- Example: 1,400-1,800 Cal/day for typical adults at complete rest
2. Non-Exercise Activity Thermogenesis (NEAT): 15-30% of daily calories
- Fidgeting, maintaining posture, daily movements, occupational activities
- Can vary by 350-600 Cal/day between individuals
- Standing burns 50 more Cal/hour than sitting
3. Thermic Effect of Food (TEF): 8-15% of calories consumed
- Energy used to digest, absorb, and process nutrients
- Protein has highest TEF (20-30%), then carbs (5-10%), then fat (0-3%)
4. Exercise: 5-15% of daily calories (for sedentary to moderately active people)
Example breakdown (2,000 Cal/day total expenditure):
- BMR: 1,400 Cal (70%)
- NEAT: 400 Cal (20%)
- TEF: 150 Cal (7.5%)
- Exercise: 50 Cal (2.5%)
Takeaway: Most calories burned come from just being alive, not exercise, though exercise remains crucial for health, fitness, and weight management.
About Kilojoule (kJ)
How do I convert kilojoules to Calories?
Formula: Calories (kcal) = kJ ÷ 4.184
Quick approximation: Divide kJ by 4 (slightly underestimates)
Examples:
- 100 kJ ≈ 24 Cal
- 500 kJ ≈ 120 Cal
- 1,000 kJ ≈ 239 Cal
- 2,000 kJ ≈ 478 Cal
- 8,400 kJ ≈ 2,007 Cal (daily intake)
Exact vs. approximate:
- 1,000 kJ ÷ 4 = 250 Cal (approximate, 4.6% high)
- 1,000 kJ ÷ 4.184 = 239 Cal (exact)
For dietary purposes, the approximate conversion (÷4) is usually sufficient.
How many kilojoules should I eat per day?
Daily kilojoule requirements depend on age, sex, body size, and activity level:
Adult women:
- Sedentary: 7,500-8,400 kJ/day (1,800-2,000 Cal)
- Moderately active: 8,400-9,600 kJ/day (2,000-2,300 Cal)
- Very active: 10,000-12,000 kJ/day (2,400-2,900 Cal)
Adult men:
- Sedentary: 8,800-10,000 kJ/day (2,100-2,400 Cal)
- Moderately active: 10,500-12,000 kJ/day (2,500-2,900 Cal)
- Very active: 12,500-14,500 kJ/day (3,000-3,500 Cal)
Children and teens: 5,000-12,500 kJ/day depending on age and activity
Athletes: 12,500-21,000 kJ/day (3,000-5,000 Cal) or more during intense training
General guideline: Most adults need 8,000-10,000 kJ daily (~2,000-2,400 Cal).
Why do some countries use kJ instead of Calories?
Scientific consistency: Kilojoules are SI units, consistent with all other scientific measurements (meters, kilograms, seconds). Calories are non-SI legacy units.
Eliminates confusion: The calorie system is inherently confusing:
- Small calorie (cal) = 4.184 J
- Large Calorie (kcal, "food Calorie") = 1,000 cal = 4,184 J
- Many people don't realize "Calories" on food labels are actually kilocalories
Kilojoules eliminate this ambiguity—1 kJ always equals 1,000 J.
International standardization: The WHO, FAO, and international scientific community use kilojoules as the standard for nutrition research and policy.
Countries using kJ:
- Exclusive kJ: Australia, New Zealand, Singapore
- Both kJ and kcal: EU (27 countries), Canada, China, Japan, India, Brazil, Argentina, South Africa
- Only kcal: United States (and a few small nations)
Result: Over 85% of the world's population sees kilojoules on food labels.
How many kilojoules do I burn exercising?
Energy expenditure varies by activity intensity, body weight, fitness level, and efficiency:
General estimates (per hour for 70 kg person):
- Walking (5 km/h): 1,200-1,500 kJ (290-360 Cal)
- Jogging (8 km/h): 2,500-3,000 kJ (600-720 Cal)
- Running (12 km/h): 3,500-4,200 kJ (840-1,000 Cal)
- Cycling (leisure): 1,200-1,600 kJ (290-380 Cal)
- Cycling (vigorous): 2,500-3,300 kJ (600-800 Cal)
- Swimming (moderate): 2,000-2,500 kJ (480-600 Cal)
- Weight training: 1,200-2,100 kJ (290-500 Cal)
- Yoga: 650-1,000 kJ (155-240 Cal)
Per-distance estimates:
- Walking: 200-250 kJ/km (50-60 Cal/km)
- Running: 300-350 kJ/km (70-85 Cal/km)
- Cycling: 100-150 kJ/km (25-35 Cal/km)
Formula (approximate): kJ/hr ≈ Body weight (kg) × MET value × 4.2
Where MET = metabolic equivalent (1 MET = resting, 3-6 = moderate, 6+ = vigorous)
Are kilojoules more accurate than Calories for nutrition?
Same accuracy, different units: Both kJ and kcal measure the same physical quantity (energy) with equal precision. The accuracy of nutrition labeling depends on measurement methods (bomb calorimetry, Atwater system), not the choice of units.
Advantages of kilojoules:
- SI consistency: Compatible with all other scientific units
- No ambiguity: 1 kJ always means 1,000 J (unlike calorie/Calorie confusion)
- International standard: Used in scientific literature worldwide
- Educational: Connects food energy to physics and chemistry
Advantages of Calories (kcal):
- Familiarity: Deeply established in US consumer culture
- Smaller numbers: 2,000 Cal/day sounds less intimidating than 8,400 kJ/day
- Historical data: Decades of dietary research in kcal
Scientific community consensus: Kilojoules are preferred for publications, while kcal persists in US consumer contexts due to inertia.
Can I lose weight by tracking kilojoules?
Yes—identical to tracking Calories: Weight loss requires a sustained energy deficit (consuming fewer kJ than you expend). Whether you count in kJ or kcal makes no difference physiologically.
Creating a deficit:
- 0.5 kg/week loss: Deficit of ~2,000 kJ/day (500 kcal/day)
- 1 kg/week loss: Deficit of ~4,000 kJ/day (1,000 kcal/day)
Practical approach:
- Calculate maintenance: Estimate daily kJ needs (8,000-12,000 kJ for most adults)
- Set deficit: Reduce intake by 1,500-2,500 kJ/day (moderate, sustainable)
- Track intake: Log food kJ using labels or apps
- Monitor progress: Adjust based on weekly weigh-ins
Example: If you need 10,000 kJ/day to maintain weight, eating 8,000 kJ/day creates a 2,000 kJ deficit = ~0.5 kg loss per week.
How accurate are kilojoule values on food labels?
Regulatory tolerances allow some variation:
Australia/NZ: ±20% tolerance for energy values (same as nutrients)
EU: Energy values must be calculated using standard conversion factors (Atwater system) or measured via bomb calorimetry. Tolerances apply.
Sources of inaccuracy:
- Natural variation: Agricultural products vary in composition
- Rounding: Labels round to nearest 10-100 kJ
- Calculation methods: Atwater factors (17 kJ/g carb, 38 kJ/g fat) are averages
- Serving size: Manufacturers sometimes understate serving sizes, making total kJ appear lower
Reality: Most packaged food kJ values are reasonably accurate (within ±10-15%), but restaurant/takeaway items may vary more significantly. Track trends over weeks rather than obsessing over individual items.
What is the relationship between kJ and metabolism?
Basal metabolic rate (BMR): The energy your body needs at rest to maintain vital functions (breathing, circulation, cell production) is typically:
- Women: 5,000-6,500 kJ/day (1,200-1,550 kcal/day)
- Men: 6,500-8,000 kJ/day (1,550-1,900 kcal/day)
Total daily energy expenditure (TDEE): BMR × activity factor
- Sedentary (little exercise): BMR × 1.2
- Lightly active (1-3 days/week): BMR × 1.375
- Moderately active (3-5 days/week): BMR × 1.55
- Very active (6-7 days/week): BMR × 1.725
- Extra active (twice daily, heavy): BMR × 1.9
Metabolic efficiency: Humans convert food energy to mechanical work at only ~25% efficiency. The other ~75% becomes heat. This is why intense exercise makes you hot!
Example: Running requires ~300 kJ/km of mechanical work, but your body actually burns ~1,200 kJ/km of food energy to produce that work (300 kJ useful, 900 kJ heat).
How do kilojoules relate to weight gain or loss?
Energy balance equation:
- Weight maintenance: Energy in (kJ consumed) = Energy out (kJ expended)
- Weight gain: Energy in > Energy out → Surplus stored as fat/muscle
- Weight loss: Energy in < Energy out → Deficit drawn from body stores
Fat energy density: 1 kg body fat ≈ 0.87 kg pure fat ≈ 33,000 kJ (7,700 kcal)
Weight loss calculation:
- To lose 0.5 kg/week: Create 4,700 kJ weekly deficit = 670 kJ/day deficit
- To lose 1 kg/week: Create 9,400 kJ weekly deficit = 1,340 kJ/day deficit
Practical example:
- Maintenance: 10,000 kJ/day
- Target: Lose 0.5 kg/week
- Daily intake: 10,000 - 670 = 9,330 kJ/day (2,230 kcal/day)
- Or: Eat 10,000 kJ but burn extra 670 kJ through exercise
Sustainable approach: Combine moderate deficit (1,500-2,000 kJ/day) with regular activity for gradual, maintainable weight loss.
Is 8,400 kJ per day a standard recommendation?
8,400 kJ (~2,000 kcal) is a common reference value for nutrition labeling (e.g., "% daily value" on labels), representing an average adult's needs. However:
Not universal: Individual needs vary by:
- Age: Older adults typically need less
- Sex: Men generally need more than women
- Body size: Larger bodies need more energy
- Activity level: Active people need significantly more
- Metabolism: Varies 10-20% between individuals
Who needs ~8,400 kJ/day?
- Sedentary to moderately active women
- Sedentary men
- Older adults
Who needs more?
- Active/very active men (10,000-14,000 kJ/day)
- Athletes (12,500-21,000+ kJ/day)
- Teenagers (especially boys)
Who needs less?
- Children (5,000-7,500 kJ/day depending on age)
- Very sedentary older adults (7,000-7,500 kJ/day)
Recommendation: Use 8,400 kJ as a starting point, then adjust based on whether you're maintaining, losing, or gaining weight.
Conversion Table: Calorie to Kilojoule
| Calorie (cal) | Kilojoule (kJ) |
|---|---|
| 0.5 | 0.002 |
| 1 | 0.004 |
| 1.5 | 0.006 |
| 2 | 0.008 |
| 5 | 0.021 |
| 10 | 0.042 |
| 25 | 0.105 |
| 50 | 0.209 |
| 100 | 0.418 |
| 250 | 1.046 |
| 500 | 2.092 |
| 1,000 | 4.184 |
People Also Ask
How do I convert Calorie to Kilojoule?
To convert Calorie to Kilojoule, enter the value in Calorie in the calculator above. The conversion will happen automatically. Use our free online converter for instant and accurate results. You can also visit our energy converter page to convert between other units in this category.
Learn more →What is the conversion factor from Calorie to Kilojoule?
The conversion factor depends on the specific relationship between Calorie and Kilojoule. You can find the exact conversion formula and factor on this page. Our calculator handles all calculations automatically. See the conversion table above for common values.
Can I convert Kilojoule back to Calorie?
Yes! You can easily convert Kilojoule back to Calorie by using the swap button (⇌) in the calculator above, or by visiting our Kilojoule to Calorie converter page. You can also explore other energy conversions on our category page.
Learn more →What are common uses for Calorie and Kilojoule?
Calorie and Kilojoule are both standard units used in energy measurements. They are commonly used in various applications including engineering, construction, cooking, and scientific research. Browse our energy converter for more conversion options.
For more energy conversion questions, visit our FAQ page or explore our conversion guides.
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Other Energy Units and Conversions
Explore other energy units and their conversion options:
- Joule (J) • Calorie to Joule
- Megajoule (MJ) • Calorie to Megajoule
- Gigajoule (GJ) • Calorie to Gigajoule
- Watt-hour (Wh) • Calorie to Watt-hour
- Kilowatt-hour (kWh) • Calorie to Kilowatt-hour
- Megawatt-hour (MWh) • Calorie to Megawatt-hour
- Kilocalorie (kcal) • Calorie to Kilocalorie
- British Thermal Unit (BTU) • Calorie to British Thermal Unit
- Therm (therm) • Calorie to Therm
- Foot-pound (ft⋅lb) • Calorie to Foot-pound
Verified Against Authority Standards
All conversion formulas have been verified against international standards and authoritative sources to ensure maximum accuracy and reliability.
National Institute of Standards and Technology — Fundamental physical constants and unit conversions
Bureau International des Poids et Mesures — International System of Units (SI) definitions and standards
International Organization for Standardization — Quantities and units international standards series
Last verified: February 19, 2026